The stock rev limiter is set at 6750rpm.
You can mechanical overspin (via momentum) the limiter by maybe 100rpm. (I.E. banging the limiter)
If you are seeing numbers past that then the tach is misreading (not a big surprise as it is Very slow responding)
The SVT's have a soft limiter of 6750rpm and hard limiter of 6900-6975rpm (each code is a little different) You can see a mechanical overspin to about 7050rpm.
Duratec Safe Limits
The safe limit of the stock components vary.
Rod bearings - maybe 7200-7300rpm (be very wary of oil level - but that's a-whole-nother story in itself)
Ring Sealing - 7600-7700rpm
"Theoretical" material limits (rods & pistons) using conventional formulas - 7650-7700rpm
"Tested" material limits - 8100rpm without failure
Valvetrain - over 8000rpm though valve float and retainer issues may arise much past 8k. However you'll never need to spin the engine this high anyway.
I would use 7200rpm as a safe limit.
If you installed better rod bearings (Clevite tri-metal) and took care of the several oiling "issues" then ~7500rpm.
Above 7500rpm the engine is in a downward spiral. Significantly increased oil consumption and wear is a given.
